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Cyclin B Dissociation from CDK1 Precedes its Degradation Upon MPF Inactivation in Mitotic Extracts of Xenopus laevis Embryos
Franck Chesnel, Franck Bazile, Aude Pascal and Jacek Z. Kubiak
volume 5 | issue 15
1 august 2006Pages: 1687 - 1698
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Cyclin B is a regulatory subunit of CDK1 within MPF complex. Degradation of cyclin B via ubiquitin-proteasome pathway seemed to be absolutely required for the M-phase exit. However, inhibition of the proteasome proteolytic activity upon the exit from the meiotic metaphase II-arrest in Xenopus cell-free extract revealed that the proteasome-dependent dissociation of cyclin B from CDK1 is sufficient to inactivate MPF without cyclin B degradation. In this study we analyse whether the same mechanism operates during the exit from mitotic M-phase. We show in Xenopus cell-free extract undergoing the first or the second embryonic mitosis that CDK1 oscillations are not affected by proteasome inhibition with MG132 or ALLN despite effective inhibition of cyclins B degradation. The majority of cyclins B1 and B2 surviving CDK1 inactivation is CDK-free and cyclin B2 becomes resistant to phosphatase ? dephosphorylation. The pool of cyclins B remaining after CDK1 inactivation in the presence of MG132 is mitotically inert, while exogenous or newly synthesised cyclin B activates CDK1. This suggests that cyclins B remain sequestered within the proteasome upon MPF inactivation in the presence of MG132. Comparison of the dynamics of the decline of total and CDK-bound pools of cyclins B1, B2 and B4 upon mitotic exit in absence of protein synthesis reveals that CDK-bound cyclins B diminish clearly faster. Our results thus show that cyclin B dissociation from CDK1 precedes cyclins B degradation upon CDK1 inactivation in mitotic embryo extracts and that proteasome proteolytic activity is dispensable for both activation and inactivation of CDK1 in such extracts.
We now provide open access to journal articles published online for one year or more. This article may be downloaded at the following link:
Download PDF If the document does not open, please right-click on the link (control-click on a Macintosh) and select the option to save the file to disk.